Jacques Fromental Halévy's spectacular La Juive - one of France's most famous and much-revived operas during the 19th century, admired by Wagner and Mahler - has fallen on hard times. While audiences at the Paris premiere in 1835, and for long afterwards, thrilled at its musical daring and were amazed by its scenic splendours, few companies seem willing to stage it today. What went wrong?

The simple answer (although not the only one) is, alas, the nature of operatic cash-flow then and now. Works premiered at the Paris Opéra in the 1830s enjoyed almost unlimited resources - for star singers, of course (there are five fiercely demanding roles), but also, and in particular, for grandiose spectacle. In La Juive's famous first-act procession, there were reputedly more than 300 magnificently attired extras on stage: the soldiers had metal armour and sat astride real horses. The ceremonial scenes in the third act were equally splendid. And, many hours later, the fifth act had as its centrepiece a huge cauldron of boiling oil, a receptacle into which the suffering heroine was duly thrown.

Rehearsals dragged on for months, with the slightest piece of stage business or twitch of an eyebrow likely to cause endless discussion and debate. Verdi, with a characteristic mixture of disapproval and envy, called the Opéra "la grande boutique" - the perfect nickname for a theatre dedicated unashamedly to the joys of conspicuous consumption.

In these days of dwindling state subsidies and stabilisation packages, an authentic re-creation of La Juive would bring even the richest opera house to its knees. Small wonder, then, that this month's Royal Opera revival is only a concert performance (which saves on horses, but also on rehearsal time). So we'll just have to imagine as best we can the glint of metal armour and the flaming cauldron.

Having the chance to look at the libretto from time to time won't hurt, though: the plot, which meanders through its five acts, is typically complicated, though packed with some reassuringly familiar operatic business. The setting is early 15th-century Constance, at the time of the famous conference that tried to come to grips with an alarming papal schism (no fewer than three popes claimed legitimacy; religious unrest was in the air). The lead character is a Jewish goldsmith called Eléazar, already the object of Christian persecution as the curtain rises. He has a daughter, Rachel (or is she really someone else's daughter?), who loves handsome young Samuel. But Samuel is really Prince Léopold in disguise, and (worse still) already married to Princess Eudoxie. To complete the team, there's a crusty old cardinal called Brogni, who - in one of the plot's several fantastic coincidences - lost his daughter many years before.

With this lineup, working out the way things go pear-shaped (or rather cauldron-shaped) is like painting by numbers. By the end of the opera, Eléazar and Rachel are awaiting execution at the hands of Brogni, who is egged on by a bloodthirsty crowd. Eléazar has hinted that he knows the whereabouts of Brogni's daughter. In the final lines, Brogni asks him again: "Where is my daughter?" just at the moment when Rachel is thrown into the flames. "La voilà!" shouts Eléazar triumphantly, and himself walks into the flaming cauldron. They don't make them like that any more.

Luckily, though, it's not just the elaborate stage business and melodramatic coups that made La Juive so popular. The opera also has some stunning musical inventions. Like all grand operas, it boasts a telling contrast between moments of big public display (with a large, hard-working chorus) and passages of intense private anguish in which the five soloists come to the fore.

The first two acts illustrate this contrast very well. The first act is predominantly public, has very little solo singing, and begins and ends with grand choral displays. The second act, on the other hand, is dedicated almost entirely to the soloists, and explores the complex personal relationships that emerge between father, daughter, the dastardly lover and his (as yet unknowing) wife. As the opera continues, though, this neat division between "public" and "private" becomes ever more difficult to maintain; some of the finest later scenes (such as the grisly finale) play out personal tragedy against a choral backdrop, with telling effect.

All five principal roles, each tailored to the peculiar talents of those warbling galacticos that only the Opéra could afford, present tremendous challenges. The aristocratic couple, Eudoxie (a coloratura soprano) and Léopold (a cruelly high tenor) are the lightest in style: neither would be out of place in a comic opera. But both also need the power to project over a noisy orchestra in the ensemble scenes. Brogni, written for the great bass Levasseur, needs far more than the usual bluster expected of old religious curmudgeons: his obsession with his lost daughter requires him, in the latter stages of the action, to show a much gentler side - a kind of emotional depth that this voice type rarely meets before Verdi. Rachel (another soprano, though with greater demands in the lower register) has several touching moments, not least her Romance in the second act, which, like most of the solo numbers, is adorned by some beautifully intricate wind and brass solos.

However, there's no doubt that the starring role is Eléazar, written for Adolphe Nourrit, the tenor who dominated the Paris stage in the 1830s. Nourrit, apparently tired of playing the bland heroes his beautiful voice usually dictated for him, wanted something more complex, and Halévy obliged with a part that goes the full gamut from quiet religious devotion to near-hysterical anger and defiance at the persecution he and his daughter suffer. It comes as no surprise that Eléazar was one of Caruso's greatest roles, or that his aria that ends the fourth act - Rachel, Quand du Seigneur - is the lyrical climax of the entire work.

Can La Juive ever regain its position as a stalwart of the repertory? In spite of its musical and dramatic successes, the likelihood is slim. Quite apart from the sheer expense of staging such lengthy and extravagant pieces, there's now something rather too studied about many of the theatrical effects of grand opera in its pure form. In this sense, it's no accident that the only two examples of the genre to have thrived in the modern repertoire are by Italians: Rossini's William Tell and Verdi's Don Carlos display all the necessary pomp and circumstance, but they also boast a much greater supply of uncomplicated melody than French composers would allow themselves, or perhaps were capable of producing.

This said, there remain good reasons to listen again to Halévy's La Juive. Its tender moments still have the power to move us; and even without the armour, the horses and the flaming cauldrons, the sheer scale of its musical and dramatic ambitions can remind us of a time, now long gone, when opera reigned supreme in the capital of the 19th century.

· La Juive is in concert at the Barbican, London EC2 on September 19 and 21. Box office: 020-7638 8891